Fan Airflow and Power Calculator
Calculates fan power consumption from blade swept area, rotational speed, and static pressure. Use it when sizing ventilation fans or verifying energy budgets for HVAC and industrial systems.
About this calculator
Fan power is determined by three interacting factors: the swept area of the blades, the rotational speed, and the pressure rise the fan must overcome. The swept area is A = π × (D/2)², where D is the fan diameter. Volumetric flow rate Q is proportional to area and tip speed. The power formula implemented here is P = A × (n/60) × ΔP × 101.6 / 1000, where n is rotational speed in RPM, ΔP is static pressure in Pa, and the constants handle unit conversion to kilowatts. Fan laws (affinity laws) state that flow scales linearly with speed, pressure scales with speed squared, and power scales with speed cubed—so doubling the RPM multiplies power by eight. Real-world efficiency losses due to blade design, turbulence, and motor inefficiency mean actual power draw will exceed this theoretical minimum.
How to use
Consider a fan with a diameter of 600 mm, spinning at 1200 RPM against a static pressure of 50 Pa. First compute the swept area: A = π × (0.3)² ≈ 0.2827 m². Then apply the formula: P = 0.2827 × (1200/60) × 50 × 101.6 / 1000 = 0.2827 × 20 × 50 × 101.6 / 1000 = 0.2827 × 101,600 / 1000 ≈ 28.73 W. This is the shaft power required at these conditions. If the motor efficiency is 85 %, the electrical input power would be 28.73 / 0.85 ≈ 33.8 W. Increasing fan speed to 2400 RPM would roughly multiply power by 2³ = 8, giving about 270 W.
Frequently asked questions
How does fan diameter affect airflow and power consumption?
Airflow is directly proportional to the swept area, which scales with the square of the diameter. Doubling the diameter quadruples the swept area and, at the same RPM, quadruples the flow rate. Power also increases with area, so a larger fan moves far more air at the same rotational speed. This is why large, slow-spinning fans are more energy-efficient than small, fast-spinning fans for high-flow, low-pressure applications—they achieve the same flow with much less turbulence and noise.
What is static pressure and why does it matter when sizing a ventilation fan?
Static pressure is the resistance the fan must overcome to push air through the duct system, filters, grilles, and any other obstructions. It is measured in Pascals and represents stored pressure energy rather than velocity energy. A fan with insufficient static pressure capability will deliver far less flow than its free-air rating suggests once installed in a real duct system. When sizing a fan, you must calculate the total system resistance at the desired flow rate and select a fan whose pressure-flow (P-Q) curve intersects the system curve at the required operating point.
What are fan affinity laws and how do they help with energy savings?
The fan affinity laws describe how flow, pressure, and power change when fan speed changes. Flow Q is proportional to speed N, static pressure ΔP is proportional to N², and shaft power P is proportional to N³. This cubic relationship with speed is extremely important for energy savings: reducing fan speed by just 20 % cuts power consumption by nearly 50 %. Variable-speed drives (VSDs) exploit this by matching fan speed to actual demand rather than running at full speed with a damper, and they typically pay back their installation cost within one to three years in commercial HVAC applications.